The integrity and function of the epithelial lining of the gastrointestinal tract, and thus human health, depend upon appropriate cellular proliferation and apoptosis, as part of the response to injury or inflammation. These processes are normally tightly regulated by an array of signaling pathways driven by cell surface receptors and their cognate ligands; aberrations in this control contribute to numerous disease states from ulceration to tumorigenesis. This project is focused on understanding the roles played by tumor necrosis factor (TNF) and its receptors (TNFR1 & TNFR2) in coordinating proliferation and apoptosis during intestinal epithelial injury and inflammation. Our studies to date have described novel mechanisms of TNFR activation and downstream signal transduction in intestinal epithelial cells, including TNF-stimulated cell survival responses such as Raf-1 regulation of nuclear factor- (NF-) B and Src-mediated transactivation of epidermal growth factor receptor (EGFR) & ErbB2. Because these receptor systems have been linked to both pathogenesis and therapeutics, defining the mechanisms of TNFR signal transduction and the role of EGFR transactivation in the gastrointestinal tract is important to understand the clinical implications, and potential applications, of these responses. Our published results from the current funding period, combined with our Preliminary Data included in this proposal, demonstrate that TNFR1 processing, TNFR2 expression, and transactivation of EGFR/ErbB2 help to maintain an intact intestinal epithelial monolayer by reducing apoptosis and promoting proliferation. Therefore, studies described in this application will test the hypothesis that during acute injury and chronic inflammation, TNF stimulates TNFR processing and signal transduction, including transactivation of EGFR/ErbB2, to promote epithelial cell monolayer integrity and health of the host. The Specific Aims are to: (1) Define the role of TNFR interactions and processing in intestinal epithelial cell response to injury in vitro and in vivo, (2) Determine the role of TNFR1 downstream anti-apoptotic signal transduction pathways in colon epithelium following acute and recurrent injury, and (3) Determine the effects of EGFR/ErbB2 on injury and repair mechanisms in acute and chronic inflammation in the colon. The proposed studies will significantly enhance our understanding of the roles and mechanisms of action of TNF-induced signaling on intestinal epithelial health, with implications for a number of human gastrointestinal disorders.